Going the Distance, Interstellar Style

You may have noticed that I have more than a few pet peeves. One of the biggest is the use of imprecise terminology in popular culture; for example, when a supposed science fiction show will use the term “intergalactic” when it’s clear that they mean “interstellar.” I mean, come on! You’re traveling between stars, not between galaxies! Gah! (Throws Cheetos at TV.)

Ahem. In any case, to coincide with the release of “Interstellar,” which seems to at least try to get it right (but fails in the end, apparently), I thought I’d use this month’s column to run down some of the more common units of distance that are used in astronomy. That might sound boring, and, well, OK… It can be. But, seriously, how often do I get to do a movie tie-in? Work with me here.

Let’s start things off at the everyday, human scales of distance that you and I are used to, and we’ll go up from there.

Meter (m):

The meter is pretty much the standard of distance in science. If you insist on being old-fashioned, that’s roughly three feet (one yard) long.

Kilometer (km):

The “kilo” prefix tells you that there are 1,000 of whatever it is you are dealing with. In this case, meters. So, a kilometer is 1,000 meters long. (That’s about 6/10 of a mile.)

Now, I will admit that, at this point, things do get a little boring. You’ve got a bunch of prefixes that mean “various powers of 10 meters,” and honestly, it will put you to sleep. Let’s take a big leap and go straight to—

Astronomical Unit (AU):

This is the distance from the Sun to the Earth. Of course, the Earth travels around the Sun in an ellipse, so the actual distance varies from day to day. So, it’s been decided that 1AU is actually 149,597,870,700 meters. (Or, roughly 150 million km.) To be sure, the AU is an entirely made-up unit of measurement and very “Earth-centric.” But it’s handy for expressing distances here in our own solar system. For example, the average distance of Pluto from the Sun is about 40AU.

Interplanetary:

This is a vague term that means the distance between the planets of a given solar system. At this point, we humans haven’t even covered this much distance yet. (Unless you count the moon as a planet, which you shouldn’t, because it isn’t.) Our probes, however, cover interplanetary distances regularly.

Interstellar:

This is another fuzzy term that means the distances between stars. Again, the distances here vary from star to star. To get more precise, you have to start talking in terms of “light-years.”

Light-Year (ly):

Yes, the word “year” is in there. But this is still a measure of distance, and not time. Specifically, it’s the distance that light travels in a year. As you may know, light is fast. In a vacuum (i.e. outer space) it travels at 299,792,458 meters per second! If you do the math, you find that, in a year, light will travel 10 trillion kilometers. So, dropping back to “interstellar” for a minute, let’s look at distances to some “nearby” stars.

Our closest stellar neighbor, Proxima Centauri, is 4.22 light-years away. That’s an interstellar distance of 42.2 trillion kilometers. The North Star, perhaps the most famous star in the sky, is a whopping 323 light-years away.

And yes, that’s still inside our galaxy. (The Milky Way is about 100,000 light years across.) But, before we talk about galaxies, we have one more unit of distance to talk about: the Parsec.

Parsec (pc):

The parsec represents a distance of 3.26 light-years. Parsecs are the main unit of measurement used in astronomy, because they take into account the motion of the Earth as it moves around the Sun.

Intergalactic:

Yet another vague term that describes the distances between galaxies. The distances here are pretty much unimaginable. Our closest galactic neighbor, the Canis Major Dwarf galaxy is “just” 25,000 light-years away. The galaxy everyone thinks is closest, the Andromeda galaxy, is actually 2.5 million light-years away.

And, there are further things still. The closest quasar to us is 13 billion light-years away...and the universe itself is about 93 billion light-years across! Oy!

So, come on, Hollywood. It’s not that hard to get this stuff right, and I’m running out of Cheetos.